I've just found one post which has awarded bounty from nobody on this answer. HTML code looks like
<span class="bounty-award" title="This answer has been awarded bounty worth 50 reputation by ">+50</span>
I have found nothing about bounty in history, even timeline has no evidence about bounty.
Is it bug?
Also, if it's merged account, it should be rollbacked. It's the same problematic guy